In a newspaper column earlier this month, Mr Duncan Smith wrote: “The acrimonious manner in which all this has been conducted is troubling and will I fear have consequences beyond June 23.”

He also questioned the integrity of cabinet colleagues campaigning to remain in the EU and accusing them of “spin, smears and threats” that will have consequences beyond the referendum.

He also wrote of “ill-spelt-out sinister threats” by the ‘in’ campaign would make Government more difficult after the vote.

Mr Grayling commented: “I’m not aware of what [Iain Duncan Smith] he said.

“I’m very grateful to the Prime Minister for allowing a free vote and I don’t think it’s something that is going to divide us.”

On Thursday Mr Cameron said: “For those who advocate leaving, lost jobs and a dented economy might be collateral damage, or a price worth paying.

“For me, they’re not. They never are. Because there’s nothing more important that protecting people’s financial security.”

Mr Grayling added: “On the jobs front, I don’t agree with him. I believe it will create jobs in the UK by getting rid of unnecessary regulation and at the same time forging new trade agreements with countries around the world.”